A:having been killed
B:have been killed
C:be killed
D:being killed
下列有关地球昼和夜的叙述,正确的是( )
图甲为一列简谐横波在某一时刻的波形图.a、b两质点的横坐标分别为x=2m和x=6m,图乙为质点b从该时刻开始计时的振动图象.下列说法正确的是( )